There are only four ingredients in most beer: Malted barley, water, hops and yeast. But on the banks of northern California’s Russian River, Stumptown Brewery’s Peter Hackett is cooking up a different kind of brew. His unique ale is made with a special ingredient: 45-million year old Saccharomyces cerevisiae (aka brewer’s yeast) rescued from a piece of amber formed during the Eocene epoch and reanimated in the lab of microbiologist Raul Cano.

The single-celled yeast, unsurprisingly robust for something that has lived 45 million years in dormancy, is shockingly good at making beer, though it’s not without its quirks. After all, modern brewer’s yeast has evolved in the anaerobic environment of a fermentation tank, while the ancient yeast hasn’t had the benefit of adapting to the harsh world inflicted by beer makers.

Here’s a look at the technical nitty-gritty behind the science and art of cooking up a batch of Fossil Fuel’s first offering: XP Ale.

Hackett’s first step in making beer starts in the attic of his brewpub, where he pours giant bags of malt into a grain mill. There, rollers crack the malt and feed it into a large tank in the brewery below. Malt is made of grains or seeds whose growth has been stunted at germination, when they contain the highest concentration of starch-digesting enzymes.

In this case, the malt is made out of barley. Beer can also be made with wheat, oats or any type of malted grain.

The mill feeds the grain at a controlled speed, along with 170-degree water, into the stainless steel mashing tub. It is stirred occasionally with a blue plastic boat oar to ensure consistent hydration of the cracked grist.

The temperature at the mash stage, says Hackett, determines the ultimate mouth-feel of the beer. This is because the heat breaks down the malt, releasing enzymes that turn the grain’s starch proteins into simple and complex sugars. Later, the yeast will consume these sugars and turn them into alcohol. The Saccharomyces cerevisiae prefers to eat simple sugars.

At this stage the liquid that will eventually be beer has a caramel color and tastes sweet like barley-flavored sugar water.

After the mash rests for an hour, Hackett circulates the liquid into and out of the tank. This step, called vorloft, removes any solids or impurities floating in the mixture. The next step is called sparging. Hackett moves the liquid, known as wort, into the kettle while adding additional hot water. This stage releases any sugars still trapped in the barley and dilutes the mixture to create a clear, nearly-flavorless fluid.

At this point the specific gravity – a measurement that determines the density of the liquid, which will be essential later in determining when the beer is finished brewing – should ideally be about 1.050 (water is 1.000).

Once in the kettle, Hackett brings the diluted wort to a violent boil.

The purpose of this is twofold. First, because the hot tanks are open to the environment, it’s possible for free-floating, sugar-loving microorganisms to get in. They’re not dangerous to eat, but they could change the flavor of the beer. Boiling pasteurizes the liquid to avoid contamination.

Its second purpose is to break out any undissolved proteins, which will appear as inch-long strings inside the kettle. The proteins will need to be removed from the wort before the yeast is added, otherwise they create what is called a protein haze — making the beer cloudy and giving it a viscous character that will affect the ultimate mouth feel.

Over the next hour, the wort will boil in the kettle and Hackett will add flavoring ingredients. Hops, which are related to cannabis and smell like marijuana, give the beer its bitter flavor.

Later additions (three total for Hackett’s special ale) will add more complex flavors and a floral aroma. Hops also act as a preservative, giving the beer a longer life span before the flavors go off and the plant’s oils contribute to the overall mouth feel of the finished beer.

When the hops addition is complete, Hackett shuts the burner off and spins the liquid inside the kettle into a whirlpool. The solids inside — the undissolved protein strands and hops — need to be removed in order to prevent them from changing the flavor of the beer and interfering with the yeast. During the whirlpool stage, the solids collect on a mound at the convex bottom of the kettle. When the liquid is removed, Hackett will transfer it out around the pack collected inside.

After 10 minutes of spinning and 20 minutes resting, the hot liquid, now at more the 190 degrees, is fully prepared and ready for the final step: turning wort into beer.

Once the liquid in the kettle has been moved to the fermentation tank Hackett cools it to about 74 degrees. Normally, this temperature would drop a bit during fermentation, but the ancient yeast likes its food to remain consistently a little warmer then other yeast.

At this stage Hackett pours a 5-gallon jug of liquid yeast (called a brink) into the side of the fermentation tank. He locks the tank shut and lets the yeast go to work.

Once the brink is emptied into the tank, the ancient yeast eats the sugars created by the mashing process and converts them into alcohol and CO2.

During the process, however, the Eocene ancestor to modern brewer’s yeast behaves differently from traditional ale yeast. It ferments quickly at the top of the tank, like an ale yeast, but then it continues to ferment slowly when it falls the bottom of the tank instead of going dormant, which is more like a lager yeast. It’s during the fast and furious top fermentation that the yeast imparts its ancient and unique flavor to the beer.

At the bottom of the tank, the ancient yeast clumps up and forms a thick clay. This is because, unlike its modern counterparts, it has an unusually high supply of agglutin — a sticky surface protein and antibody that causes the yeast’s cell walls to become compact, highly concentrated, and rigid in an effort to prevent foreign bodies from getting in.

After about a week in the fermentation tub, Hackett “crashes the tank” by dropping the temperature. Normally this would shock ale yeast into dormancy. But the ancient yeast will continue to eat, so Hackett moves it to a conditioning tank where the beer will sit and slowly ferment for another month. When the specific gravity and flavor profile is just right, the beer is done.

A standard ale, made with modern hybridized brewing yeasts, will start at a higher specific gravity. For example, a regular ale will have a starting gravity of 1.058 to 1.065 and would finish at about 1.014.

The ancient yeast, however, only eats simple sugars and not as much as modern yeast. Because of this, Hackett has to start at a lower specific gravity, 1.050, and it will finish at about 1.012.

Hackett, who likes to think of himself as the Research and Development end of Fossil Fuels Brewing Co. has made other variations of beer with the ancient yeast. Stumptown Brewery was recently selling a strong, dark caramel, Scottish brew Hackett called “XPort.”

But his original XP, also known as Experimental Pale Ale, is his pride and joy. “You would have better odds of winning the lottery,” Hackett said of the likelihood that the ancient yeast would brew tasty beer. “I was very surprised to find that it did a great job. I was over the moon.”